This invention relates to optical fibers and, more specifically, to double-clad optical fibers, particularly as they are used in cladding-pumped optical amplifiers and lasers.
An optical amplifier is a device that increases the amplitude of an input optical signal fed thereto. If the optical signal at the input to such an amplifier is monochromatic, the output will also be monochromatic, with the same frequency. A conventional fiber amplifier comprises a gain medium, such as a glass fiber core doped with an active material, into which is coupled to an input signal. Excitation occurs from the absorption of optical pumping energy by the core. The optical pumping energy is within the absorption band of the active material in the core, and when the optical signal propagates through the core, the absorbed pump energy causes amplification of the signal transmitted through the fiber core by stimulated emission. Optical amplifiers are typically used in a variety of applications including but not limited to amplification of weak optical pulses such as those that have traveled through a long length of optical fiber in communication systems.
One typical example of a fiber amplifier is referred to as an erbium (Er) amplifier, and includes a silica fiber having a single-mode core doped with erbium (specifically doped with erbium ions conventionally denoted as Er3+). It is well known that an erbium optical fiber amplifier operating in its standard so-called three level mode is capable, when pumped at a wavelength of 980 nanometers (nm), of amplifying optical signals having a wavelength of 1550 nm. Since 1550 nm is the lowest loss wavelength of conventional single-mode silica glass fibers, erbium amplifiers are well suited for inclusion in fiber systems that propagate signals having wavelengths around 1550 nm.
In certain applications, particularly high-power ones, it may be desirable to provide optical amplification using a double-clad fiber. A typical double-clad fiber has an inner core, through which an optical signal is transmitted, an inner cladding surrounding the core that is of lower refractive index than the core, and an outer cladding surrounding the inner cladding that has a lower refractive index than the inner cladding. When using a double-clad fiber for optical amplification, it is known that the optical pumping energy need not be coupled directly into the core, where it will be absorbed for amplification purposes, but may be coupled into the inner cladding, where it propagates in various reflective trajectories through the cladding until it intersects the core. Once contacting the core, pump energy is absorbed and provides stored energy in the core for stimulated emission amplification of the optical signal.
One well-known problem with the use of double-clad fibers for optical amplifiers is the fact that among the transmission modes of the pumping energy through the inner cladding are a number which never intersect the core. Because a conventional double-clad fiber has a cylindrical core surrounded by an annular inner cladding, it is possible for a number of helical modes to exist within the inner cladding which travel through the inner cladding without ever intersecting the core. Since these modes never intersect the core, the pump energy is not absorbed and does not contribute to the amplification of the optical signal. This has led to attempts to reduce the helical spatial modes within the inner cladding of a double-clad fiber.
One way of reducing the problem with modes that do not intersect the core is to promote xe2x80x9cmode mixing.xe2x80x9d By changing the shape of the inner cladding, the number of reflective helical modes within the inner cladding can be minimized. In short, the introduction of different surface variations into the cross-sectional shape of the inner cladding results in reflective patterns through the cladding that must relatively quickly intersect the core. Such a design is shown in U.S. Pat. No. 4,815,079 to Snitzer et al. In FIG. 2 of the Snitzer et al. patent, a cross-sectional view of a fiber shows an inner cladding 210 with a rectangular shape. This cladding relies on a difference in its width and height to provide the desired mode mixing within. However, its oblong shape is difficult to produce, and limits the end coupling into the fiber.
Another prior art cladding design is shown in International Patent Application WO 97/12429 to Zellmer et al. FIG. 2 of this application depicts the problem with helical modes in a double-clad fiber having a conventional inner cladding. Zellmer et al. address the problem of these helical modes by attempting to promote mode mixing by introducing a flat section into the otherwise circular cross section of the inner cladding outer surface. This inner cladding shape is shown in FIG. 3 of the application. This approach does appear to improve mode mixing over the traditional fiber shape, and is relatively simple to produce compared to a rectangular construction. Since only one flat surface has to be formed, only a single region of a fiber preform must be removed from the inner cladding of a fiber with a circular cross section, a structure that is relatively easy to make. In the description of the application, the length of the flat region is described as being 1% to 49% of the diameter of the inner cladding.
The prior art described above is directed toward mode mixing to increase the coupling of pumping energy from the inner cladding into the core. However, while the methods described do enhance mode mixing, they do so by significantly distorting the shape of the inner cladding. Thus, the end coupling profile of the inner cladding is significantly altered relative to a typical double-clad fiber with a circular inner cladding cross section. This reduces the capacity for coupling pump energy into or out of the altered fiber shape.
The present invention provides a double-clad optical fiber having an inner cladding with a cross-sectional shape that not only induces mode mixing, but also preserves a profile that is equal in perpendicular dimensions. That is, the inner cladding has a cross-sectional shape such that two perpendicular distances across the shape, each of which passes through a geometric center of the core, are equal for all angular positions. Thus, the inner cladding is not oblong in any particular dimension and, no particular cross-sectional dimension of the inner cladding has a preference for coupling capacity.
The fiber includes a core through which an optical signal propagates. The core is surrounded by the inner cladding, which has a lower index of refraction than the core. The inner cladding is, in turn, surrounded by an outer cladding that has an index of refraction lower than the inner cladding. Herein, reference to the xe2x80x9ccross-sectional shapexe2x80x9d of the inner cladding refers to the shape of its outer surface in a plane perpendicular to a longitudinal direction in which optical signal energy propagates through the core of the fiber.
In a first embodiment of the invention, the cross-sectional shape of the inner cladding includes two flat surfaces, a first of which is colinear with a first line and a second of which is colinear with a second line that is perpendicular to the first line. Thus, the two flat surfaces are perpendicular to each other, and are located, relative to a geometric center of the core in the cross-sectional plane, at an angle of 90 relative to each other. In a variation of this embodiment, the cross-sectional shape has four flat surfaces separated by other surfaces of the shape, wherein two of the four flat surfaces are parallel to a first line and two are parallel to a second line, the first and second lines preferably being perpendicular to each other. In still another variation of the primary embodiment, a cross-sectional shape of the inner cladding of the fiber is octagonal.
The fiber of the embodiment described above may be used in a fiber amplifier in which a first optical signal in a first wavelength band travels through a core of the fiber in a longitudinal direction. The core is doped with an active element such that it absorbs light in a second wavelength band and outputs light in the first wavelength band in response to the first optical signal propagating through the core. An optical pumping source is used to generate an optical pumping signal in the second wavelength band that is coupled into the inner cladding of the fiber. The pumping signal may be coupled into an end of the fiber, taking advantage of the profile of the inner cladding, or may be side coupled into a flat region along the outer surface of the inner cladding.
In an alternative embodiment of the invention, a double-clad optical fiber has a core through which optical energy travels in a longitudinal direction, surrounded by an inner cladding which, in turn, is surrounded by an outer cladding. In this embodiment, the cross-sectional shape of the inner cladding may be circular. However, the inner cladding has a torsional stress induced by rotation of the fiber during manufacture about the longitudinal axis of the fiber. Preferably, after applying the inner cladding layer to the core, and prior to curing it in an oven, a rotation is imparted to the fiber which causes the torsional stress in the inner cladding material. When the fiber is thereafter cured, the torsional stress becomes permanently fixed in the inner cladding layer. This stress disrupts the helical modes within the inner cladding, and increases the level of mode mixing.